1. Field of the Invention
This invention relates to a method of rapidly determining the acceptability of a load of devices to be sterilized by chemical sterilization with a germicide gas or vapor, for example, with hydrogen peroxide.
2. Description of the Related Art
The need to sterilize articles such as medical devices is well known. There are many methods of sterilization, including heat and chemical methods. Heat sterilization is normally done with steam. The heat and/or moisture from the steam can damage sensitive medical devices. As a result, chemical sterilization is commonly used to minimize damage to medical devices during sterilization.
Chemical sterilization uses a sterilizing fluid such as hydrogen peroxide, ethylene oxide, chlorine dioxide, formaldehyde, or peracetic acid in a sealed chamber to sterilize medical instruments. One commercial form of chemical sterilization is the STERRAD(copyright) Sterilization System, available through Advanced Sterilization Products of Irvine, Calif., a division of Ethicon, Inc. The STERRAD Process utilizes hydrogen peroxide and low temperature gas plasma to sterilize medical devices.
The STERRAD Sterilization Process is performed in the following manner. The items to be sterilized are placed in a sterilization chamber, the chamber is closed, and a vacuum is drawn. An aqueous solution of hydrogen peroxide is injected and vaporized into the chamber. A low-temperature gas plasma is initiated by applying radio frequency energy to create an electric field. The hydrogen peroxide vapor dissociates in the plasma into reactive species that react with and kill microorganisms. After the activated components react with the organisms or with each other, they lose their high energy and recombine to form oxygen, water, and other nontoxic byproducts. At the completion of the process, the RF energy is turned off, the vacuum is released, and the chamber is returned to atmosphenc pressure by venting.
In order for the sterilization process to be effective, sufficient hydrogen peroxide must be introduced into the chamber. If the equipment in the chamber reacts with, absorbs, adsorbs, or condenses the hydrogen peroxide, there may not be sufficient hydrogen peroxide remaining in the chamber for the sterilization process to be effective. The concentration of hydrogen peroxide vapor in the chamber is therefore monitored to assure that sufficient hydrogen peroxide is present. If too much hydrogen peroxide is removed from the chamber through absorption, adsorption, condensation, or reaction with the equipment in the chamber, the cycle is canceled, the remaining hydrogen peroxide in the chamber is removed by evacuating the chamber and/or introducing plasma to decompose the hydrogen peroxide, and a new cycle is started.
Cummings et al. (U.S. Pat. No. 4,956,145) describe a method in which the hydrogen peroxide concentration is monitored, and additional hydrogen peroxide is added to maintain the concentration of hydrogen peroxide at a level which is effective for sterilization but is less than the saturation limit. Cummings et al. did not describe any method for determining whether the equipment in the sterilization chamber significantly absorbs, adsorbs, condenses, or decomposes large amounts of hydrogen peroxide, however. If hydrogen peroxide is absorbed, adsorbed, or condensed onto the equipment, it may take a great deal of time to remove the hydrogen peroxide so that the equipment may be safely removed from the chamber.
There is a need for a method for rapidly determining whether the load of equipment in the chamber is suitable or not so that the sterilization cycle can be canceled before significant amounts of hydrogen peroxide have been absorbed, adsorbed, or condensed onto the equipment. If the unsuitability of the load could be determined rapidly, the cycle can be aborted and the hydrogen peroxide removed from the chamber before significant amounts of hydrogen peroxide have been absorbed, adsorbed, or condensed by the equipment in the chamber. In this manner, the length of time required to abort the cycle and start a new cycle can be minimized.
One aspect of the invention involves a method for determining the suitability of a load for sterilization with a germicide vapor or gas including placing the load into a sterilization chamber, evacuating the sterilization chamber, and contacting the load in the sterilization chamber with the germicide vapor or gas. The method also includes monitoring the concentration of the germicide vapor or gas in the sterilization chamber as a function of time, determining the rate of change of the concentration of the germicide vapor or gas in the sterilization chamber, and determining the suitability of the load from the rate of change. The load is suitable for sterilization if the rate of change is less than an empirically-derived rate at which a pre-determined level of sterilization is achieved.
Advantageously, the pre-determined level of sterilization is a reduction in microorganisms to a level of 10xe2x88x926 of the initial level. Preferably, the suitability of the load is determined less than 100 seconds after contacting the load with the germicide vapor or gas. In a preferred embodiment, the rate of change in the concentration is determined as the initial slope of a graph of logx(c/co) versus time, where x is any number, c is the concentration of the germicide gas or vapor, and co is the maximum concentration of the germicide gas or vapor in the sterilization chamber.
Preferably, logx is log10 or ln. Advantageously, the germicide gas or vapor is hydrogen peroxide vapor. In a preferred embodiment, the load is contacted with plasma before the load is contacted with hydrogen peroxide vapor. In an embodiment, the load is determined to be unsuitable if the negative of the initial slope is 0.016 secxe2x88x921 or greater, where logx is ln. Advantageously, the sterilization is aborted if the load is determined to not be suitable. Aborting may be performed by evacuating the sterilization chamber or by generating plasma in the sterilization chamber.
In another embodiment, more hydrogen peroxide may be added into the sterilization chamber, if the load is determined to be unsuitable. Advantageously, monitoring can be performed by measuring the concentration of hydrogen peroxide vapor with a spectrometer or by measuring the quantity of heat evolved by a reaction of the hydrogen peroxide vapor with a chemical compound.
Another aspect of the invention involves a method for rapidly determining the suitability of a load for sterilization with a germicide vapor or gas including placing the load into a sterilization chamber, evacuating the sterilization chamber, contacting the load in the sterilization chamber with germicide vapor or gas, and monitoring the concentration of the germicide vapor or gas in the sterilization chamber as a function of time. The method also includes determining the area under a curve of a graph of c versus time, where c is the concentration of the germicide vapor or gas. The method also includes determining the suitability of the load, where the load is suitable for sterilization if the area is greater than an empirically derived area with which a pre-determined level of sterilization is achieved.
Preferably, the germicide vapor or gas includes hydrogen peroxide. Advantageously, the area under the curve is determined from the time of the maximum concentration of the germicide vapor or gas to a time at the end of contacting the load with germicide vapor or gas. In an embodiment, the load is suitable if the area under the curve is greater than 400 mg-sec/L. In a preferred embodiment, the sterilization is aborted, if the load is determined to be not suitable.
Another aspect of the invention involves a method for rapidly determining the suitability of a load for sterilization with a germicide vapor or gas including placing the load into a sterilization chamber, evacuating the sterilization chamber, contacting the load in the sterilization chamber with germicide vapor or gas, and determining the concentration of the germicide vapor or gas. The method also includes determining the suitability of the load from the concentration, where the load is suitable for sterilization if the concentration is greater than an empirically derived concentration with which a pre-determined level of sterilization is achieved.
Preferably, the sterilizalion is aborted if the load is determined to be not suitable. Advantageously, the germicide vapor or gas includes hydrogen peroxide. In a preferred embodiment, the load is suitable for sterilization if the concentration of germicide vapor or gas is 0.47 mg/L or greater.